1. Field of the Invention
The present invention generally relates to a film formation method, and more particularly to a method for forming a tungsten nitride film for a semiconductor device.
2. Description of the Related Art
In many cases, when forming a thin film using a conventionally known thermal CVD (Chemical Vapor Deposition) method, a gas that acts as a reactant is supplied to a reaction chamber in which a heated substrate has been mounted, and a thermal chemical reaction is caused either in the space inside the chamber or on the substrate surface, thereby depositing a film onto the substrate.
This method is used for forming films of metals (such as, W (tungsten) or Al (aluminum)), or insulation films (such as, SiO2 (silicon dioxide)), which has been used on semiconductor devices to form wiring or interlayer insulation films.
Recently, nitride films (such as, TiN (titanium nitride) or WN (tungsten nitride)) have started to be used as contact layers for use with W, or barrier films for use with Cu wiring. However, with the increasing miniaturization of the structures of semiconductor devices, ever higher levels of performance are being demanded of the thin films formed by CVD methods.
For example, when forming a titanium nitride film, the ability to form the film uniformly on the side walls of contact holes with aspect ratios exceeding 10 is required. With nitride based barrier films, there are other requirements including the ability to form a uniform film with a film thickness of no more than 5 nm on the side walls of via holes, while also maintaining conventional levels of contact resistance. However, it is difficult to form films which satisfy this level of performance using conventional thermal CVD methods.
Furthermore, if Cu wiring is formed on the substrate, then heating the substrate to high temperatures exceeding 350° C. may cause voids in the Cu wiring; and consequently, in order to maintain the reliability of the Cu wiring, film formation is preferably performed under conditions where the substrate temperature is no more than 350° C., and even more preferably no more than 300° C.
Among conventional barrier film formation methods, a method has been proposed in which a material gas (such as, tungsten fluoride gas, a nitriding gas which nitrides the material gas, and a reducing gas which reduces the material gas) are all supplied to the same reaction chamber, thereby forming a barrier film comprising tungsten nitride (W×N). With this method it is possible to form the barrier film under low temperature conditions of no more than 500° C. (For example, see Japanese Patent Laid-Open Publication No. 2001-23930.)
However, this film formation method also requires that the substrate is heated to a temperature of at least 350° C., and when the method is actually applied, film formation is performed with the substrate heated to approximately 380° C., and consequently forming the film while maintaining good reliability of the Cu wiring is problematic.
Furthermore, another method for forming a barrier film comprising a metal nitride, in which film formation is performed by alternately supplying a material gas comprising a metal-containing gas, and a nitriding gas such as ammonia, into a reaction chamber, has also been proposed. (For example, see Japanese Patent Laid-Open Publication No. Hei 11-54459.)
When a tungsten nitride film is formed by this method, the film having a high content of nitrogen and excellent adhesion to silicon substrates can be formed, as in WN and W2N or the like, but because the high resistance of the film makes the film unsuited to use as a barrier film and further the deposition rate is slow, this method is difficult to apply in practical situations.
In addition, a method in which a barrier film is formed by alternately supplying a material gas comprising a metal-containing gas, and hydrogen gas, to a reaction chamber has also been suggested in the conventional technology. (For example, see Japanese Patent No. 3,415,207 or Japanese Patent Laid-Open Publication No. Hei 6-89873.)
If film formation is performed in accordance with this method, using a tungsten-containing gas, then a film with extremely high tungsten content can be formed at a fast deposition rate. Films with a high tungsten content have excellent adhesion to Cu, as well as low resistance. However, because such films have poor adhesion to SiO2, the films may come off the substrate when formed on a silicon substrate.
Other publications of the conventional technology for the present invention include Japanese Patent Laid-Open Publication No. 2000-212749, Japanese Patent Laid-Open Publication No. 2001-319930, and Japanese Patent Laid-Open Publication No. Hei 7-252660.
The present invention was developed in order to solve the problems with the conventional technology described above, with an object of providing a method for forming a tungsten nitride film which can be applied to barrier film formation and to film formation in contact holes with high aspect ratios.